The present invention relates to heating, ventilation and air conditioning (HVAC) systems for vehicles, and in particular to such systems having dual modes for providing heat to passenger compartments of the vehicles. 
In a conventional automotive vehicle employing an internal combustion engine, the heating of the passenger compartment is accomplished by running engine coolant, typically a mix of water and glycol (antifreeze), through a heater core in the passenger compartment, and then blowing air over the heater core and onto the passengers. The drawback with this is that the heater core will not provide heat until the engine has caused the coolant to warm up. For most conventional engines, this time to warm up the coolant is sufficiently short to satisfy the vehicle passengers. 
Now, however, newer engines and powertrain arrangements are being developed where the engine does not produce as much excess heat for the coolant to absorb. Some examples are a direct injection engine and a hybrid (engine/motor) powertrain. For these types of powertrains, the temperature of the coolant can take a very long time to rise to a level where it will allow for adequate heating of the passenger compartment when using a conventional heating system. 
Most automotive vehicles today also include an air conditioning system for cooling the air in the passenger compartment. The air conditioning system can begin to operate almost as soon as the vehicle is started. Some, then, have recognized that the components of the air conditioning system can be employed to operate in a heat pump mode, and so the conventional coolant based heating system is supplemented or replaced with heat from the heat pump operation. But these systems become less and less efficient for heating as the heat source temperature becomes colder. 
Moreover, since these types of combination air conditioning/heat pump systems employ the air conditioning system evaporator as a condenser in heat pump mode, there can be a flash fogging concern that occurs when the system is switched from operating in the air conditioning mode to operating in the heat pump mode. That is, when operating in the air conditioning mode, water may condense on the  evaporator. Then, when the system is switched to the heat pump mode, the evaporator now acts as a condenser, causing the water condensate to vaporize. Also, the air flowing in the heat pump mode is not dehumidified because it is heated rather than cooled before passing through the heater core. Blowing the humid air on the windshield can possibly cause fogging on a cold windshield, which is generally disliked by vehicle occupants. 
Another limitation to the air conditioning/heat pump types of systems is that they need to be capable of performing this additional function—over and above a conventional vehicle HVAC system—while minimizing the additional cost of such a system. Thus minimizing the cost of additional components is desirable. 
Thus, it is desirable to have a vehicle heating and cooling system that overcomes the drawbacks of conventional vehicle heating, as well as typical heat pump systems, in order to warm a vehicle passenger compartment more quickly, while minimizing the additional cost. 